This invention relates to Al—Zn—Mg—Cu alloys and more particularly it relates to Al—Zn—Mg—Cu extrusions and the method of making the same for use in aircraft applications. Further, the invention relates to Al—Zn—Mg—Cu alloy extrusion product having improved fracture toughness.
Existing Al—Zn—Mg—Cu alloys can have relatively high strengths at moderate corrosion resistance and moderate damage tolerance or fracture toughness. Such alloys and methods of obtaining properties are set forth in the patents. For example, U.S. Pat. No. 4,863,528 discloses a method for producing an aluminum alloy product and the resulting product having improved combinations of strength and corrosion resistance. The method includes providing an alloy consisting essentially of about 6–16% zinc, about 1.5–4.5% magnesium, about 1–3% copper, one or more elements selected from zirconium, chromium, manganese, titanium, vanadium and hafnium, the total of said elements not exceeding about 1%, the balance aluminum and incidental impurities. The alloy is then solution heat treated; precipitation hardened to increase its strength to a level exceeding the as-solution heat treated strength level by at least about 30% of the difference between as-solution heat treated strength and peak strength; subjected to treatment at a sufficient temperature or temperatures for improving its corrosion resistance properties; and again precipitation hardened to raise its yield strength and produce a high strength, highly corrosion resistant alloy product.
U.S. Pat. No. 5,221,377 discloses an alloy product having improved combinations of strength, density, toughness and corrosion resistance, said alloy product consisting essentially of about 7.6 to 8.4% zinc, about 1.8 to 2.2% magnesium, about 2 to 2.6% copper and at least one element selected from zirconium, vanadium and hafnium present in a total amount not exceeding about 0.5%, preferably about 0.05 to 0.25% zirconium, the balance aluminum and incidental elements and impurities. The alloy product, suitable for aerospace applications, exhibits high yield strength, at least about 10% greater yield strength than its 7X50-T6 counterpart, with good toughness and corrosion resistance properties typically comparable to or better than those of its 7X50-T76 counterpart. Upper wing members made from this alloy typically have a yield strength over 84 ksi, good fracture toughness and an EXCO exfoliation resistance level of “EC” or better, typically “EB”.
U.S. Pat. No. 4,477,292 discloses a three-step thermal aging method for improving the strength and corrosion resistance of an article comprising a solution heat treated aluminum alloy containing zinc, magnesium, copper and at least one element selected from the group consisting of chromium, manganese and zirconium. The article is precipitation hardened at about 175° to 325° F., heat treated for from several minutes to a few hours at a temperature of about 360° to 390° F. and again precipitation hardened at about 175° to 325° F. In a preferred embodiment the article treated comprises aluminum alloy 7075 in the T6 condition. The method of the invention is easier to control and is suitable for treating articles of greater thickness than other comparable methods.
U.S. Pat. No. 5,108,520 discloses an aging process for solution-heat-treated, precipitation hardening metal alloy which includes first underaging the alloy, such that a yield strength below peak yield strength is obtained, followed by higher aging for improving the corrosion resistance of the alloy, followed by lower temperature aging to strength increased over that achieved initially.
U.S. Pat. No. 5,560,789 discloses AA 7000 series alloys having high mechanical strength and a process for obtaining them. The alloys contain, by weight, 7 to 13.5% Zn, 1 to 3.8% Mg, 0.6 to 2.7% Cu, 0 to 0.5% Mn, 0 to 0.4% Cr, 0 to 0.2% Zr, others up to 0.05% each and 0.15% total, and remainder Al. Either wrought or cast alloys can be obtained, and the specific energy associated with the DEA melting signal of the product is lower than 3 J/g.
U.S. Pat. No. 5,312,498 discloses a method of producing an aluminum-based alloy product having improved exfoliation resistance and fracture toughness which comprises providing an aluminum-based alloy composition consisting essentially of about 5.5–10.0% by weight of zinc, about 1.75–2.6% by weight of magnesium, about 1.8–2.75% by weight of copper with the balance aluminum and other elements. The aluminum-based alloy is worked, heat treated, quenched and aged to produce a product having improved corrosion resistance and mechanical properties. The amounts of zinc, magnesium and copper are stoichiometrically balanced such that after precipitation is essentially complete as a result of the aging process, no excess elements are present. The method of producing the aluminum-based alloy product utilizes either a one- or two-step aging process in conjunction with the stoichiometrically balancing of copper, magnesium and zinc.
U.S. Pat. No. 4,711,762 discloses an improved aluminum base alloy product comprising 0 to 3.0 wt. % Cu, 0 to 1.5 wt. % Mn, 0.1 to 4.0 wt. % Mg, 0.8 to 8.5 wt. % Zn, at least 0.005 wt. % Sr, max. 1.0 wt. % Si, max. 0.8 wt. % Fe and max. 0.45 wt. % Cr, 0 to 0.2 wt. % Zr, the remainder aluminum and incidental elements and impurities.
U.S. Pat. No. 1,418,303 discloses an improved aluminum alloy consisting of copper about 0.1% to any amount below 3%, titanium about 0.1% to about 2%, zinc about 6% to about 16%, iron (present as an impurity of commercial aluminum) preferably not exceeding 0.6%, silicon (present as an impurity of commercial aluminum) preferably not exceeding 0.4%, other elements (impurities) preferably not exceeding 0.4%, remainder aluminum.
U.S. Pat. No. 2,290,020 discloses an improved aluminum alloy having the ternary compound of aluminum, zinc and magnesium present in an amount ranging from about 2% to 20%, the preferred range being between about 3% and 15%. At room temperature the ternary compound goes into solid solution in aluminum alloys in an amount of about 2%. The percentage in solid solution increases at high temperatures and decreases upon cooling, the excess precipitating out.
U.S. Pat. No. 3,637,441 discloses an aluminum base powder metallurgy alloy article having an improved combination of high-transverse yield strength and high-stress corrosion cracking resistance. The alloy contains the basic precipitation hardening elements zinc, magnesium and copper plus dispersion strengthening elements iron and nickel. It may additionally contain chromium and/or manganese. The alloy is prepared by atomization of a melt of the elements, hot-working, solution heat treating, quenching and artificial aging. Components of the alloy in percent by weight are, in addition to the aluminum, from at least 6.5 to 13 zinc, 1.75 to 6 magnesium, 0.25 to 2.5 copper, 0.75 to 4.25 iron and 0.75 to 6 nickel, up to 3 manganese and up to 0.75 chromium. The iron to nickel ratio is from 0.2:1 to 2.0:1.
U.S. Pat. No. 5,028,393 discloses an Al-based alloy for use as sliding material, superior in fatigue resistance and anti-seizure property consisting, by weight, of 1–10% Zn, 1–15% Si, 0.1–5% Cu, 0.1–5% Pb, 0.005–0.5% Sr, and the balance Al and incidental impurities.
U.S. Pat. No. 6,315,842 discloses a mold for plastics made of a rolled, extruded or forged AlZnMgCu aluminum alloy product >60 mm thick, and having a composition including, in weight %: 5.7<Zn<8.7, 1.7<Mg<2.5, 1.2<Cu<2.2, Fe<0.14, Si<0.11, 0.05<Zr<0.15, Mn<0.02, Cr<0.02, with Cu+Mg<4.1 and Mg>Cu, other elements <0.05 each and <0.10 in total, the product being treated by solution heat treating, quenching and aging to a T6 temper.
In spite of these discloses, there is still a great need for an improved alloy and extrusion fabricated therefrom for aerospace applications having high levels of strength, corrosion resistance, fracture toughness and good resistance to fatigue crack growth. The subject invention provides such an extrusion.